The present invention relates to a sensor for measuring a differential pressure of liquids and the like.
As a sensor for measuring a pressure of a liquid, use is made of a diaphragm-type pressure sensor so as to avoid a sensor portion being adversely affected by a liquid to be measured. As a method for measuring a differential pressure of two liquids, there is a method of using two diaphragm-type pressure sensors, in which a differential pressure is determined from a difference between outputs of the two sensors. In this method, however, a measurement error is likely to occur when the characteristics of the two sensors are not truly identical.
U.S. Pat. No. 6,640,640B2 and U.S. Pat. No. 6,431,003B1 disclose a sensor comprising two diaphragms disposed so as to face each other and which are connected to each other by means of a rod. In this sensor, a pressure applied to one diaphragm is transmitted to the other diaphragm. Therefore, only a difference between the pressures applied to the two diaphragms can be detected. In U.S. Pat. No. 6,640,640B2 and U.S. Pat. No. 6,431,003B1, piezo resistance elements are respectively provided in the two diaphragms and a difference in resistance value between the two piezo resistance elements is provided as an output of the sensor, or an electrode is provided so as to face the two diaphragms to thereby form a capacitor between the electrode and each of the two diaphragms, and a change in capacitance of the capacitor is detected to provide an output of the sensor. In these techniques, when the pressures acting on the diaphragms increase to a great extent, the diaphragms are subject to intricate deformation, as shown in FIG. 1. In such a case, an amount of deformation of each of the diaphragms exceeds a level corresponding to a differential pressure to be detected. For example, in a case that a 50 kPa full-scale differential pressure sensor is used with a baseline pressure of 300 kPa, deformation such as that shown in FIG. 1 significantly influences a resistance value of the piezo element provided in the diaphragm or a capacitance value of the capacitor between the diaphragm and the corresponding surface of the electrode. Although both of the diaphragms are subject to intricate deformation, it is extremely difficult to achieve the completely equal levels of influence of intricate deformation of each of the two diaphragms. Consequently, a zero output of the sensor shifts easily. Further, highly accurate and sensitive detection of a differential pressure is difficult in a method using a piezo resistance element directly attached to the deforming diaphragm or a method of conducting direct, electrical conversion of a change in capacitance of a capacitor formed by the deforming diaphragm.
Generally, as a method for detecting an amount of deformation of the diaphragm, there is a method of detecting a change in resistance value of a piezo element provided in part of the diaphragm. There is also a capacitor-type method using a diaphragm made of a metal, in which an electrode is provided so as to face the diaphragm, and in which a change in capacitance of a capacitor formed between the diaphragm and the electrode is detected. Although the capacitor-type method requires a relatively complex electrical system, the capacitor-type method is free from problems such as thermal noise, and is stable in terms of a temperature, due to the principles of operation. Therefore, the capacitor-type method is suitably applied to a high-precision diaphragm-type pressure sensor.
FIG. 2 is a diagram for explaining an operation of a conventional capacitor-type differential pressure sensor. Disk-like electrodes E1 and E2 are disposed so as to face metal diaphragms D1 and D2, respectively, to thereby form capacitors C1 and C2. This sensor is adapted to detect a difference between pressures P1 and P2 as a difference between capacitances of the capacitors C1 and C2.
The present inventor has made intensive studies with respect to a change in capacitance of the capacitors C1 and C2 due to a pressure applied to the diaphragms D1 and D2. As a result, it has been found that a variance in output of the capacitors and a shift of a zero point as shown in FIG. 3 are obtained. From this result, it has been found that due to a rise in pressure applied to the opposing surfaces of the diaphragms, capacitances of the capacitors C1 and C2 change, and there is a slight difference between an amount of change in capacitance of the capacitor C1 and an amount of change in capacitance of the capacitor C2. That is, even when the same pressure is applied to the two diaphragms, there is a slight difference in deformation between the two diaphragms, and the capacitances of the capacitors C1 and C2 become different from each other, thus changing an output of the sensor.